Ap (áp-) is the Vedic Sanskrit term for "water", which in Classical Sanskrit only occurs in the plural, āpas (sometimes re-analysed as a thematic singular, āpa-), whence Hindiāp. The term is from PIEhxap "water". The Indo-Iranian word also survives as the Persian word for water, āb, e.g. in Punjab (from panj-āb "five waters"). In archaic ablauting contractions, the laryngeal of the PIE root remains visible in Vedic Sanskrit, e.g. pratīpa- "against the current", from *proti-hxp-o-.

In the Rigveda, several hymns are dedicated to "the waters" (āpas): 7.49, 10.9, 10.30, 10.47. In the oldest of these, 7.49, the waters are connected with the drought of Indra. Agni, the god of fire, has a close association with water and is often referred to as Apām Napāt "offspring of the waters". The female deity Apah is the presiding deity of Purva Ashadha (The former invincible one) asterism in Vedic astrology